The present invention relates generally to subterranean cementing operations, and more particularly, to methods of preparing improved settable compositions free of Portland cement and methods of using such compositions in subterranean operations.
Hydraulic cement compositions commonly are utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions often may be used in primary cementing operations whereby pipe strings, such as casings and liners, are cemented in well bores. In performing primary cementing, hydraulic cement compositions may be placed within an annular space between the walls of a well bore and the exterior surface of a pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming therein an annular sheath of hardened substantially impermeable cement that supports and positions the pipe string in the well bore, and that bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.
A broad variety of hydraulic cement compositions have been used in conventional subterranean cementing operations, including, inter alia, cement compositions comprising Portland cement. Portland cement is generally prepared from a mixture of raw materials comprising calcium oxide, silicon oxide, aluminium oxide, ferric oxide, and magnesium oxide. The mixture of the raw materials is heated in a kiln to approximately 3000° F., thereby initiating chemical reactions between the raw materials. In these reactions, crystalline compounds, dicalcium silicates, tricalcium silicates, tricalcium aluminates, and tetracalcium aluminoferrites, are formed. The product of these reactions is known as a clinker. The addition of gypsum to the clinker and the pulverization of the mixture results in a fine powder that will react to form a slurry upon the addition of water.
There are drawbacks, however, to the conventional preparation and use of Portland cement. The energy requirements to produce Portland cement are quite high, and heat loss during production can further cause actual energy requirements to be even greater. These factors contribute significantly to the relatively high cost of Portland cement. Generally, Portland cement is a major component of the cost of hydraulic cement compositions that comprise Portland cement. Recent Portland cement shortages, however, have further contributed to the rising cost of hydraulic cement compositions that comprise Portland cement. Conventional efforts to reduce the cost of cement compositions have included incorporating other solid particulate components in the cement composition in addition to, or in place of, the Portland cement. The resulting combination of multiple solid particulate materials in the cement compositions often results in a “settling effect,” in which different sized particulate materials will settle separately at different speeds. This settling of the solids in a cement composition has resulted in defective cementing and failure of the set cement to provide zonal isolation.